The older individual constitutes an increasingly large proportion of the population in the USA. The extremes of fatness or leanness which are regulated by fluctuations in energy balance are important health problems in aging man. Regular physical activity is routinely prescribed for the older individual to aid in weight control as well as increase the quality of life and functional independence, yet little is known regarding the effects of variations in physical activity on food intake and resting energy metabolism. This proposal will address the effects of fluctuations in physical activity on 3 outcome variables in old and young men: 1) spontaneous food intake (FI), 2) resting metabolic rate (RMR) and norepinephrine (NE) kinetics as an indicator of sympathetic nervous system activity (SNSa). The approach to be taken is as follows: Study 1 will cross-sectionally examine 4 groups of men (active and inactive, old and young) to determine the effects of age and physical activity status on RMR and SNSa in a 10 day baseline. Study 2A will examine the response of changing physical activity levels in old and young men with no previous history of regular physical activity of FI, RMR and SNSa. Volunteers will participate in a 24 day program of increased physical activity (2 week outpatient, 10 day inpatient) and will be restudied after a 24 day period of resumed inactivity. Adaptations of ACTIVE old and young men to variations in physical activity may be different than those of INACTIVE men. Study 2B will therefore examine changes in FI, RMR and SNSa in old and young men with a history of regular participation in physical activity and restudied after 24 days of resumed regular physical activity. In experiments 2A and 2B, we will also validate the factorial method with the doubly labeled water method to estimate total energy expenditure. These studies will provide new information on how physically active and inactive lifestyles regulate appetite and resting energy metabolism in old and young men.